U.S. patent number 6,234,634 [Application Number 09/363,256] was granted by the patent office on 2001-05-22 for image projection system with a polarizing beam splitter.
This patent grant is currently assigned to Moxtek. Invention is credited to Eric Gardner, Douglas P. Hansen, Mark W. Lund, Raymond T. Perkins.
United States Patent |
6,234,634 |
Hansen , et al. |
May 22, 2001 |
Image projection system with a polarizing beam splitter
Abstract
An image projection system has a wire grid polarizing beam
splitter which functions as both the polarizer and the analyzer in
the system. A light source produces a source light beam directed at
the beam splitter which reflects one polarization and transmits the
other. A liquid crystal array is disposed in either the reflected
or transmitted beam. The array modulates the polarization of the
beam, encoding image information thereon, and directs the modulated
beam back to the beam splitter. The beam splitter again reflects
one polarization and transmits the other so that the encoded image
is either reflected or transmitted to a screen. The wire grid
polarizing beam splitter is capable of being oriented at various
incident angles with respect to the source light beam and modulated
beam, and accepts relatively divergent light.
Inventors: |
Hansen; Douglas P. (Spanish
Fork, UT), Perkins; Raymond T. (Orem, UT), Gardner;
Eric (Provo, UT), Lund; Mark W. (Orem, UT) |
Assignee: |
Moxtek (Orem, UT)
|
Family
ID: |
23429487 |
Appl.
No.: |
09/363,256 |
Filed: |
July 28, 1999 |
Current U.S.
Class: |
353/20 |
Current CPC
Class: |
G02B
27/283 (20130101); G02B 5/3058 (20130101) |
Current International
Class: |
G02B
27/28 (20060101); G02B 5/30 (20060101); G03B
021/14 () |
Field of
Search: |
;353/20,122,31,33,34,37
;349/8,9 ;359/486 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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416157 |
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Jul 1925 |
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DE |
|
296391 |
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Feb 1950 |
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DE |
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3707984A1 |
|
Mar 1987 |
|
DE |
|
0317910A1 |
|
Nov 1987 |
|
EP |
|
0349309B1 |
|
Jun 1988 |
|
EP |
|
0349144B1 |
|
Jun 1988 |
|
EP |
|
0357946B1 |
|
Aug 1988 |
|
EP |
|
0336334B1 |
|
Aug 1988 |
|
EP |
|
0407830B1 |
|
Jul 1989 |
|
EP |
|
0407830A2 |
|
Jul 1989 |
|
EP |
|
0416157A1 |
|
Sep 1989 |
|
EP |
|
0488544A1 |
|
Nov 1990 |
|
EP |
|
0507445A2 |
|
Mar 1991 |
|
EP |
|
0518111A1 |
|
May 1991 |
|
EP |
|
0588937B1 |
|
Jun 1991 |
|
EP |
|
0521591B1 |
|
Jul 1991 |
|
EP |
|
0543061A1 |
|
Nov 1991 |
|
EP |
|
0606940A2 |
|
Jan 1993 |
|
EP |
|
0634674A2 |
|
Jun 1993 |
|
EP |
|
0670506A1 |
|
Sep 1993 |
|
EP |
|
0744634A2 |
|
May 1996 |
|
EP |
|
0084502 |
|
Aug 1989 |
|
JP |
|
1781659A1 |
|
Oct 1990 |
|
SU |
|
Other References
Lloyd William Taylor Manual of Advanced Undergraduate Experiments
in Physics, p. 302 (1959). .
Flanders, Application of .apprxeq. 100 .ANG. linewidth structures
fabricated by shadowing techniques.sup.a), J. Vac. Sci. Technol.,
19(4), Nov./Dec. 1981. .
Kuta and van Driel, "Coupled-wave analysis of lamellar metal
transmission gratings for the visible and the infrared," J. Opt.
Soc. Am. A/vol. 12, No. 5/May 1995. .
Lockbihler and Depine, "Diffraction from highly conducting wire
gratings of arbitrary cross-section," Journal of Modern Optics,
1993, vol. 40, No. 7, pp. 1273-1298. .
Novak et al., "Far infrared polarizing grids for use at cryogenic
temperatures," Applied Optics, Aug. 15, 1989/vol. 28, No. 15, pp.
3425-3427. .
Auton and Hutley, "Grid Polarizers for Use in the Near Infrared,"
Infrared Physics, 1972, vol. 12, pp. 95-100. .
Stenkamp et al, "Grid polarizer for the visible spectral region,"
SPIE vol. 2213 pp. 288-296. .
Handbook of Optics, 1978, pp. 10-68 -10-77. .
Handbook of Optics vol. II, 2.sup.nd Edition, pp. 3.32-3.35. .
Glytsis and Gaylord, "High-spatial-frequency binary and multilevel
stairstep gratings: polarization-selective mirrors and broadband
antireflection surfaces," Applied Optics Aug. 1, 1992, vol. 31, No.
22 pp. 4459-4470. .
Auton, "Infrared Transmission Polarizers by Photolithography,"
Applied Optics Jun. 1967 vol. 6, No. 6, pp. 1023-1027. .
Haggans et al., "Lamellar gratings as polarization components for
specularly reflected beams," Journal of Modern Optics, 1993, vol.
40, No. 4, pp. 675-686. .
Bird and Parrish, Jr., "The Wire Grid as a Near-Infrared
Polarizer," Lasers in Industry, pp. 886-891 (1972). .
Optics 9.sup.th Edition, pp. 338-339 (1980). .
Whitbourn and Douglas, "Phase shifts in transmission line models of
thin periodic metal grids," Applied Optics Aug. 15, 1989 vol. 28,
No. 15, pp. 3511-3515. .
Enger and Case, "Optical elements with ultrahigh spatial-frequency
surface corrugations," Applied Optics Oct. 15, 1983, vol. 22, No.
20 pp. 3220-3228. .
Knop, "Reflection Grating Polarizer for the Infrared," Optics
Communications vol. 26, No. 3, Sep. 1978. .
Hass and O'Hara, "Sheet Infrared Transmission Polarizers," Applied
Optics Aug. 1965, vol. 4, No. 8 pp. 1027-1031. .
Flanders, "Submicron periodicity gratins as artificial anisotropic
dielectrics," Apr. Phys. Lett. 42 (6), Mar. 15, 1983, pp. 492-494.
.
Li Li and J.A. Dobrowski, "Visible broadband, wide-angle, thin-film
multilayer polarizing beam splitter," Applied Optics May 1, 1996,
vol. 35, No. 13, pp. 2221-2224. .
Sonek et al., "Ultraviolet grating polarizers," J. Vac. Sci.
Technol., 19(4), Nov./Dec. 1981, pp. 921-923..
|
Primary Examiner: Dowling; William
Attorney, Agent or Firm: Thorpe North & Western LLP
Claims
What is claimed is:
1. An image projection system for providing a bright and clear
image, the system comprising:
a light source producing a visible light beam;
a polarizing beam splitter located near the light source in the
light beam and oriented at an angle with respect to the light beam,
the beam splitter comprising
a transparent substrate having a first surface located in the light
beam with the light beam striking the first surface at an angle,
and
a generally parallel arrangement of thin, elongated elements
supported by the substrate, the arrangement being configured and
the elements being sized to interact with electromagnetic waves of
the source light beam to generally (i) transmit light through the
elements which has a polarization oriented perpendicular to a plane
that includes at least one of the elements and the direction of the
incident light beam, defining a transmitted beam, and (ii) reflect
light from the elements which has a polarization oriented parallel
with the plane that includes at least one of the elements and the
direction of the incident light beam, defining a reflected
beam;
a reflective array located near the polarizing beam splitter in
either the reflected or transmitted beam, the array modulating the
polarization of the beam by selectively altering the polarization
of the beam to encode image information thereon and creating a
modulated beam, the array being oriented to direct the modulated
beam back towards the polarizing beam splitter;
the beam splitter further being located in the modulated beam and
oriented at an angle with respect to the modulated beam, and the
arrangement of elements of the beam splitter interacting with
electromagnetic waves of the modulated beam to generally (i)
transmit light through the elements which has a polarization
oriented perpendicular to the plane that includes at least one of
the elements and the direction of the incident light beam, defining
a second transmitted beam, and (ii) reflect light from the elements
which has a polarization parallel with the plane that includes at
least one of the elements and the direction of the incident light
beam, defining a second reflected beam, to separate out the
unaltered polarization from the modulated beam;
a screen located in either the second reflected beam or the second
transmitted beam for displaying the encoded image information.
2. The system of claim 1, wherein the beam splitter is a generally
planar sheet.
3. The system of claim 1, wherein the beam splitter is oriented
with respect to the light beam or the modulated beam at an incident
angle between approximately 0 to 80 degrees.
4. The system of claim 1, wherein the beam splitter is oriented
with respect to the light beam or the modulated beam at incidence
angles greater than 47 degrees or less than 43 degrees.
5. The system of claim 1, wherein the light beam has a useful
divergent cone with a half angle between approximately 12 and
25.degree..
6. The system of claim 1, wherein the beam splitter is used at an
F-number less than approximately f/2.5.
7. The system of claim 1, wherein the beam splitter has a
throughput of at least 50% defined by the product of the fractional
amount of p-polarization transmitted light and the fractional
amount of s-polarization reflected light; and wherein the
s-polarization transmitted light and p-polarization reflected light
are both less than 5%.
8. The system of claim 1, wherein the beam splitter has a
throughput of at least 50% defined by the product of the fractional
amount of s-polarization transmitted light and the fractional
amount of p-polarization reflected light; and wherein the
p-polarization transmitted light and s-polarization reflected light
are both less than 5%.
9. The system of claim 1, wherein the beam splitter has a
throughput for the light beam of at least 65%, defined by the
product of the fractional amount of reflected light and the
fractional amount of transmitted light; and wherein the percent of
reflected light or the percent of transmitted light is greater than
approximately 67%.
10. The system of claim 1, further comprising a pre-polarizer
disposed between the light source and the beam splitter.
11. The system of claim 1, further comprising a post-polarizer
disposed between the beam splitter and the screen.
12. The system of claim 1, wherein the array is disposed in the
reflected beam; and wherein the screen is disposed in the second
transmitted beam.
13. The system of claim 1, wherein the array is disposed in the
transmitted beam; and wherein the screen is disposed in the second
reflected beam.
14. The system of claim 1,
wherein the arrangement of elements has a period less than
approximately 0.21 microns,
wherein the elements have a thickness between approximately 0.04 to
0.5 microns, and
wherein the elements have a width of between approximately 30 to
76% of the period.
15. The system of claim 1, wherein the elements each have a cross
section with a base, a top opposite the base, and opposite left and
right sides; and wherein the sides form an angle with respect to
the base greater than approximately 68 degrees.
16. An image projection system comprising:
a light source producing a visible light beam having a wavelength
between approximately 0.4 and 0.7 microns;
a polarizing beam splitter located near the light source in the
light beam and oriented at an angle with respect to the light beam,
the beam splitter comprising
a transparent substrate having a surface located in the light beam
with the light beam striking the surface at an angle, and
a generally parallel arrangement of thin, elongated elements
supported by the substrate, the arrangement being configured and
the elements being sized to interact with electromagnetic waves of
the source light beam to generally (i) transmit light through the
elements which has a polarization oriented perpendicular to a plane
that includes at least one of the elements and the direction of the
incident light beam, defining a transmitted beam, and (ii) reflect
light from the elements which has a polarization oriented parallel
with the plane that includes at least one of the elements and the
direction of the incident light beam, defining a reflected
beam;
a reflective array located near the polarizing beam splitter in the
reflected beam, the array modulating the polarization of the
reflected beam by selectively altering the polarization of the
reflected beam to encode image information thereon and creating a
modulated beam, the array being oriented to direct the modulated
beam back to the polarizing beam splitter;
the polarizing beam splitter further being located in the modulated
beam and oriented at an angle relative to the modulated beam, the
arrangement of elements of the beam splitter interacting with
electromagnetic waves of the modulated beam to generally (i)
transmit light through the elements which has a polarization
oriented perpendicular to a plane that includes at least one of the
elements and the direction of the incident light beam, defining a
transmitted beam, and (ii) reflect light from the elements which
has a polarization parallel with the plane that includes at least
one of the elements and the direction of the incident light beam,
to separate out the unaltered polarization from the modulated beam;
and
a screen disposed in the transmitted beam for displaying the
encoded image information.
17. The system of claim 16, wherein the beam splitter is oriented
with respect to the light beam or the modulated beam at an incident
angle between approximately 0 to 80 degrees.
18. The system of claim 16, wherein the light beam has a divergence
cone with a useful half angle between approximately 12 and
25.degree..
19. The system of claim 16, wherein the beam splitter is used at an
F-number less than approximately f/2.5.
20. The system of claim 16, wherein the beam splitter has a
throughput for the light beam of at least 65%, defined by the
product of the fractional amount of reflected light and the
fractional amount of transmitted light; and wherein the fractional
amount of reflected light or the fractional amount of the
transmitted light of the modulated beam is greater than
approximately 0.67.
21. The system of claim 16,
wherein the arrangement of elements has a period less than
approximately 0.21 microns,
wherein the elements have a thickness between approximately 0.04 to
0.5 microns, and
wherein the elements have a width of between approximately 30 to
76% of the period.
22. An image projection system comprising:
a light source producing a visible light beam having a wavelength
between approximately 0.4 and 0.7 microns;
a polarizing beam splitter located near the light source in the
light beam and oriented at an angle with respect to the light beam,
the beam splitter comprising
a transparent substrate having a surface located in the light beam
with the light striking the surface at an angle, and
a generally parallel arrangement of thin, elongated elements
supported by the substrate, the arrangement being configured and
the elements being sized to interact with electromagnetic waves of
the source light beam to generally (i) transmit light through the
elements which has a polarization oriented perpendicular to the
plane that includes at least one of the elements and the direction
of the incident light beam, defining a transmitted beam, and (ii)
reflect light from the elements which has a polarization oriented
parallel with the plane that includes at least one of the elements
and the direction of the incident light beam;
a reflective array located near the polarizing beam splitter in the
transmitted beam, the array modulating the polarization of the
transmitted beam by selectively altering the polarization of the
transmitted beam to encode image information thereon and creating a
modulated beam, and being oriented to direct the modulated beam
back to the beam splitter;
the polarizing beam splitter further being located in the modulated
beam and oriented at an angle with respect to the modulated beam,
the arrangement of elements of the beam splitter interacting with
electromagnetic waves of the modulated beam to generally (i)
transmit light through the elements which has a polarization
oriented perpendicular to a plane that includes at least one of the
elements and the direction of the incident light beam, and (ii)
reflect light from the elements which has a polarization parallel
with the plane that includes at least one of the elements and the
direction of the incident light beam, defining a reflected beam, to
separate the altered polarization from the unaltered polarization
in the modulated beam, thereby extracting the image information in
the modulated beam; and
a screen located in the reflected beam for displaying the extracted
image information in the modulated beam.
23. The system of claim 22, wherein the beam splitter is oriented
with respect to the light beam or the modulated beam at an incident
angle between approximately 0 to 80 degrees.
24. The system of claim 22, wherein the light beam has a divergent
cone with a useful half angle between approximately 12 and
25.degree..
25. The system of claim 22, wherein the beam splitter is used at an
F-number less than approximately f/2.5.
26. The system of claim 22, wherein the beam splitter has a
throughput for the light beam of at least 65%, defined by the
product of the fractional amount of reflected light and the
fractional amount of transmitted light; and wherein the fractional
amount of reflected light or the fractional amount of transmitted
light is greater than approximately 0.67.
27. The system of claim 22,
wherein the arrangement of elements has a period less than
approximately 0.21 microns,
wherein the elements have a thickness between approximately 0.04 to
0.5 microns, and
wherein the elements have a width of between approximately 30 to
76% of the period.
28. A method for projecting an image, the method comprising:
producing a source light beam having a wavelength in a range
between approximately 0.4 to 0.7 microns using a light source;
substantially separating the polarizations of the source light beam
using a polarizing beam splitter disposed in the source light beam,
the polarizing beam splitter comprising
a generally parallel arrangement of thin, elongated elements
configured and sized to interact with electromagnetic waves of the
source light beam to generally (i) transmit light through the
elements which has a polarization oriented perpendicular to a plane
that includes at least one of the elements and the direction of the
incident light beam, defining a transmitted beam, and (ii) reflect
light from the elements which has a polarization orientation that
lies in the plane that includes at least one of the elements and
the direction of the incident light beam, defining a reflected
beam;
modulating either the transmitted or reflected beam and creating a
modulated beam by selectively altering the polarization of the beam
using an array disposed in either the transmitted or reflected
beam;
substantially separating the polarizations of the modulated beam
using the polarizing beam splitter disposed in the modulated beam,
the elements interacting with electromagnetic waves of the
modulated beam to generally (i) transmit light through the elements
which has a polarization oriented perpendicular to plane that
includes at least one of the elements and the direction of the
incident light beam, defining a second transmitted beam, and (ii)
reflect light from the elements which has a polarization
orientation that lies in the plane that includes at least one of
the elements and the direction of the incident light beam, defining
a second reflected beam; and
displaying either the second transmitted beam or the second
reflected beam on a screen.
29. An image display system for producing a visible image, the
system comprising:
a light source for emitting a source light beam having a wavelength
in a range between approximately 0.4 to 0.7 microns;
a liquid crystal array positioned and oriented for receiving and
modulating at least a portion of the source light beam and creating
a modulated beam, the modulated beam containing image
information;
a screen positioned and oriented for receiving and displaying at
least a portion of the modulated beam; and
a polarizing beam splitter disposed in both the source light beam
and the modulated beam, the beam splitter being used at an F-number
less than approximately f/2.5, the polarizing beam splitter
comprising
a generally parallel arrangement of thin, elongated elements
configured and sized to interact with electromagnetic waves of the
source light beam to generally (i) transmit light through the
elements which has a polarization oriented perpendicular to a plane
that includes at least one of the elements and the direction of the
incident light beam, defining a transmitted beam, and (ii) reflect
light from the elements which has a polarization orientation that
lies in the plane that includes at least one of the elements and
the direction of the incident light beam, defining a reflected
beam, and interacts with the electromagnetic waves of the modulated
beam to generally (i) transmit light through the elements which has
a polarization oriented perpendicular to a plane that includes at
least one of the elements and the direction of the modulated light
beam, defining a second transmitted beam, and (ii) reflect light
from the elements which has a polarization orientation that lies in
the plane that includes at least one of the elements and the
direction of the modulated light beam, defining a second reflected
beam; and
wherein the beam splitter is oriented with respect to the source
light beam at an angle of incidence between approximately 10 to 80
degrees.
30. The system of claim 29,
wherein the arrangement of elements has a period less than
approximately 0.21 microns,
wherein the elements have a thickness between approximately 0.04 to
0.5 microns, and
wherein the elements have a width of between approximately 30 to
76% of the period.
31. The system of claim 29, wherein the array is disposed in the
reflected beam, and wherein the screen is disposed in the second
transmitted beam.
32. The system of claim 29, wherein the array is disposed in the
transmitted beam, and wherein the screen is disposed in the second
reflected beam.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image projection system
operable within the visible spectrum which includes a polarizing
beam splitter which reflects one linear polarization of light and
transmits the other. More particularly, the present invention
relates to such an image projection system with a compact,
lightweight beam splitter that is comprised of a plurality of
elongated, reflective elements for interacting with the
electromagnetic waves of the source light to generally transmit or
pass one polarization of light, and reflect the other
polarization.
2. Prior Art
Polarized light is necessary in certain applications, such as
projection liquid crystal displays (LCD). Such a display is
typically comprised of a light source; optical elements, such as
lenses to gather and focus the light; a polarizer that transmits
one polarization of the light to the liquid crystal array; a liquid
crystal array for manipulating the polarization of the light to
encode image information thereon; means for addressing each pixel
of the array to either change or retain the polarization; a second
polarizer (called an analyzer) to reject the unwanted light from
the selected pixels; and a screen upon which the image is
focused.
It is possible to use a single polarizing beam splitter (PBS) to
serve both as the first polarizer and the second polarizer
(analyzer). If the liquid crystal array is reflective, for example
a Liquid Crystal On Silicon (LCOS) light valve, it can reflect the
beam that comes from the polarizer directly back to the polarizer
after encoding the image by modifying the polarization of selected
pixels. Such a system was envisioned by Takanashi (U.S. Pat. No.
5,239,322). The concept was elaborated by Fritz and Gold (U.S. Pat.
No. 5,513,023). These similar approaches would provide important
advantages in optical layout and performance. Neither, however, has
been realized in practice because of deficiencies in conventional
polarizing beam splitters. The disadvantages of using conventional
polarizing beam splitters in projection liquid crystal displays
includes images that are not bright, have poor contrast, and have
non-uniform color balance or non-uniform intensity (due to
non-uniform performance over the light cone). In addition, many
conventional polarizing beam splitters are short-lived because of
excessive heating, and are very expensive.
In order for such an image projection system to be commercially
successful, it must deliver images which are significantly better
than the images provided by conventional cathode ray tube (CRT)
television displays because it is likely that such a system will be
more expensive than conventional CRT technology. Therefore, the
image projection system must provide (1) bright images with the
appropriate colors or color balance; (2) have good image contrast;
and (3) be as inexpensive as possible. An improved polarizing beam
splitter (PBS) is an important part of achieving this goal because
the PBS is a limiting component which determines the potential
performance of the display system.
The PBS characteristics which significantly affect the display
performance are (1) the angular aperture, or the f-number, at which
the polarizer can function; (2) the absorption, or energy losses,
associated with the use of the PBS; and (3) the durability of the
PBS. In optics, the angular aperture or f-number describes the
angle of the light cone which the PBS can use and maintain the
desired performance level. Larger cones, or smaller f-numbers, are
desired because the larger cones allow for more light to be
gathered from the light source, which leads to greater energy
efficiency and more compact systems.
The absorption and energy losses associated with the use of the PBS
obviously affect the brightness of the system since the more light
lost in the optics, the less light remains which can be projected
to the view screen. In addition, the amount of light energy which
is absorbed by the polarizer will affect its durability, especially
in such image projection systems in which the light passing through
the optical system is very intense, on the order of watts per
square centimeter. Light this intense can easily damage common
polarizers, such as Polaroid sheets. In fact, the issue of
durability limits the polarizers which can be used in these
applications.
Durability is also important because the smaller and lighter the
projection system can be made, the less expensive and more
desirable is the product. To accomplish this goal, however, the
light intensity must be raised even higher, further stressing the
PBS, and shortening its useful life.
A problematic disadvantage of conventional PBS devices is poor
conversion efficiency, which is the primary critical performance
factor in displays. Conversion efficiency is a measure describing
how much of the electrical power required by the light source is
translated into light intensity power on the screen or panel that
is observed by people viewing it. It is expressed as the ratio of
total light power on the screen divided by the electrical power
required by the light source. The conventional units are lumens per
watt. A high ratio is desirable for a number of reasons. For
example, a low conversion efficiency will require a brighter light
source, with its accompanying larger power supply, excess heat,
larger enclosures and cabinet, etc. In addition, all of these
consequences of low conversion efficiency raise the cost of the
projection system.
A fundamental cause of low conversion efficiency is poor optical
efficiency, which is directly related to the f-number of the
optical system. A system which has an f-number which is half the
f-number of an otherwise equivalent system has the potential to be
four times as efficient in gathering light from the light source.
Therefore, it is desirable to provide an improved polarizing beam
splitter (PBS) which allows more efficient harvesting of light
energy by offering a significantly smaller potential f-number
(larger angular aperture), and therefore increases the conversion
efficiency, as measured in lumens/watt.
There are several reasons for the poor performance of conventional
polarizing beam splitters with respect to conversion efficiency
when they are used as beam splitters in projection systems. First,
current beam splitters work poorly if the light does not strike
them at a certain angle (or at least, within a narrow cone of
angles about this principal angle of incidence). Deviation of the
principal ray from this angle causes each type of polarizing beam
splitter to degrade the intensity, the purity of polarization,
and/or the color balance. This applies to the beam coming from the
light source as well as to the beam reflected from the liquid
crystal array. This principal angle depends upon the design and
construction of the PBS as well as the physics of the polarization
mechanism employed in these various beam splitters. Currently
available polarizing beam splitters are not capable of operating
efficiently at angles far from their principal polarizing angles in
the visible portion of the electromagnetic spectrum. This
restriction makes it impossible to implement certain promising
optical layouts and commercially promising display designs.
Even if the principal ray strikes the polarizer at the best angle
for separating the two polarizations, the other rays cannot diverge
far from this angle or their visual qualities will be degraded.
This is a serious deficiency in a display apparatus because the
light striking the polarizer must be strongly convergent or
divergent to make efficient use of the light emitted by typical
light sources. This is usually expressed as the f-number of the
optical system. For a single lens, the f-number is the ratio of the
aperture to the focal length. For optical elements in general, the
F-number is defined as
where n is the refractive index of the space within which the
optical element is located, and .theta. is the half cone angle. The
smaller the F-number, the greater the radiant flux, .phi..sub.c,
collected by the lens, and the more efficient the device will be
for displaying a bright image. The radiant flux increases as the
inverse square of the F/#. In an optical train, the optical element
with the largest F/# will be the limiting factor in its optical
efficiency. For displays using traditional polarizers, the limiting
element is nearly always the polarizer, and thus the PBS limits the
conversion efficiency. It would clearly be beneficial to develop a
type of PBS that has a smaller F/# than any that are currently
available.
Because traditional polarizers with small F/#s have not been
available, designers typically have addressed the issue of
conversion efficiency by specifying a smaller, brighter light
source. Such sources, typically arc lamps, are available, but they
require expensive power supplies that are heavy, bulky, and need
constant cooling while in operation. Cooling fans cause unwanted
noise and vibration. These features are detrimental to the utility
of projectors and similar displays. Again, a PBS with a small F/#
would enable efficient gathering of light from low-power, quiet,
conventional light sources.
Another key disadvantage of conventional polarizing beam splitters
is a low extinction, which results in poor contrast in the image.
Extinction is the ratio of the light transmitted through the
polarizer of the desired polarization to the light rejected of the
undesired polarization. In an efficient display, this ratio must be
maintained at a minimum value over the entire cone of light passing
through the PBS. Therefore, it is desirable to provide a polarizing
beam splitter which has a high extinction ratio resulting in a high
contrast image.
A third disadvantage of conventional polarizing beam splitters is a
non-uniform response over the visible spectrum, or poor color
fidelity. The result is poor color balance which leads to further
inefficiency in the projection display system as some light from
the bright colors must be thrown away to accommodate the weaknesses
in the polarizing beam splitter. Therefore, it is desirable to
provide an improved polarizing beam splitter that has a uniform
response over the visible spectrum, (or good color fidelity) giving
an image with good color balance with better efficiency. The beam
splitter must be achromatic rather than distort the projected
color, and it must not allow crosstalk between the polarizations
because this degrades image acuity and contrast. These
characteristics must apply over all portions of the polarizer and
over all angles of light incidence occurring at the polarizer. The
term spathic has been coined (R. C. Jones, Jour. Optical Soc. Amer.
39, 1058, 1949) to describe a polarizer that conserves
cross-sectional area, solid angle, and the relative intensity
distribution of wavelengths in the polarized beam. A PBS that
serves as both a polarizer and analyzer must be spathic for both
transmission and reflection, even in light beams of large angular
aperture.
A fourth disadvantage of conventional polarizing beam splitters is
poor durability. Many conventional polarizing beam splitters are
subject to deterioration caused by excessive heating and
photochemical reactions. Therefore, it is desirable to provide an
improved polarizing beam splitter that can withstand an intense
photon flux for thousands of hours without showing signs of
deterioration. In addition, it is desirable to provide a polarizing
beam splitter that is amenable to economical large scale
fabrication.
The need to meet these, and other, criteria has resulted in only a
few types of polarizers finding actual use in a projection system.
Many attempts have been made to incorporate both wide angular
aperture and high fidelity polarization into the same beam
splitting device. The relative success of these efforts is
described below. Thin film interference filters are the type of
polarizer cited most frequently in efforts to make a polarizing
beam splitter that is also used as an analyzer. MacNeille was the
first to describe such a polarizer that was effective over a wide
spectral range (U.S. Pat. No. 2,403,731). It is composed of
thin-film multi-layers set diagonally to the incident light,
typically within a glass cube, so it is bulky and heavy compared to
a sheet polarizer. What is more, it must be designed for a single
angle of incidence, usually 45.degree., and its performance is poor
if light is incident at angles different from this by even as
little as 2.degree.. Others have improved on the design (e.g. J.
Mouchart, J. Begel, and E. Duda, Applied Optics 28, 2847-2853,
1989; and L. Li and J. A. Dobrowolski, Applied Optics 13,
2221-2225, 1996). All of them found it necessary to seriously
reduce the wavelength range if the angular aperture is to be
increased. This can be done in certain designs (U.S. Pat. Nos.
5,658,060 and 5,798,819) in which the optical design divides the
light into appropriate color bands before it arrives at the
polarizing beam splitter. In this way, it is possible to reduce the
spectral bandwidth demands on the beam splitter and expand its
angular aperture, but the additional components and complexity add
significant cost, bulk, and weight to the system.
Even so, these improved beam splitter cubes are appearing on the
market, and are currently available from well known vendors such as
Balzers and OCLI. They typically offer an F/# of f/2.5-f/2.8, which
is a significant improvement over what was available 2 years ago,
but is still far from the range of F/1.2-F/2.0 which is certainly
within reach of the other key components in optical projection
systems. Reaching these f-numbers has the potential to improve
system efficiency by as much as a factor of 4. They would also
enable the projection display engineer to make previously
impossible design trade-offs to achieve other goals, such as
reduced physical size and weight, lower cost, etc.
In a technology far from visible optics, namely radar, wire grids
have been used successfully to polarize long wavelength radar
waves. These wire grid polarizers have also been used as
reflectors. They are also well known as optical components in the
infrared (IR), where they are used principally as transmissive
polarizer elements.
Although it has not been demonstrated, some have postulated
possible use of a wire grid polarizer in display applications in
the visible portion of the spectrum. For example, Grinberg (U.S.
Pat. No. 4,688,897) suggested that a wire grid polarizer serve as
both a reflector and an electrode (but not simultaneously as an
analyzer) for a liquid crystal display.
Others have posed the possible use of a wire grid polarizer in
place of a dichroic polarizer to improve the efficiency of virtual
image displays (see U.S. Pat. No. 5,383,053). The need for contrast
or extinction in the grid polarizer, however, is explicitly
dismissed, and the grid is basically used as a polarization
sensitive beam steering device. It does not serve the purpose of
either an analyzer, or a polarizer, in the U.S. Pat. No. 5,383,053.
It is also clear from the text that a broadband polarizing cube
beam splitter would have served the purpose as well, if one had
been available. This technology, however, is dismissed as being too
restricted in acceptance angle to even be functional, as well as
prohibitively expensive.
Another patent (U.S. Pat. No. 4,679,910) describes the use of a
grid polarizer in an imaging system designed for the testing of IR
cameras and other IR instruments. In this case, the application
requires a beam splitter for the long wavelength infra-red, in
which case a grid polarizer is the only practical solution. The
patent does not suggest utility for the visible range or even
mention the need for a large angular aperture. Neither does it
address the need for efficient conversion of light into a viewable
image, nor the need for broadband performance.
Other patents also exist for wire-grid polarizers in the infrared
portion of the spectrum (U.S. Pat. Nos. 4,514,479, 4,743,093; and
5,177,635, for example). Except for the exceptions just cited, the
emphasis is solely on the transmission performance of the polarizer
in the IR spectrum.
These references demonstrate that it has been known for many years
that wire-grid arrays can function generally as polarizers.
Nevertheless, they apparently have not been proposed and developed
for image projection systems. One possible reason that wire grid
polarizers have not been applied in the visible spectrum is the
difficulty of manufacture. U.S. Pat. No. 4,514,479 teaches a method
of holographic exposure of photoresist and subsequent etching in an
ion mill to make a wire grid polarizer for the near infrared
region; in U.S. Pat. No. 5,122,907, small, elongated ellipsoids of
metal are embedded in a transparent matrix that is subsequently
stretched to align their long axes of the metal ellipsoids to some
degree. Although the transmitted beam is polarized, the device does
not reflect well. Furthermore, the ellipsoid particles have not
been made small enough to be useful in the visible part of the
electromagnetic spectrum. Accordingly, practical applications have
been generally limited to the longer wavelengths of the IR
spectrum.
Another prior art polarizer achieves much finer lines by grazing
angle evaporative deposition (U.S. Pat. No. 4,456,515).
Unfortunately, the lines have such small cross sections that the
interaction with the visible light is weak, and so the optical
efficiency is too poor for use in the production of images. As in
several of these prior art efforts, this device has wires with
shapes and spacings that are largely random. Such randomness
degrades performance because regions of closely spaced elements do
not transmit well, and regions of widely spaced elements have poor
reflectance. The resulting degree of polarization (extinction) is
less than maximal if either or both of these effects occur, as they
surely must if placement has some randomness to it.
For perfect (and near perfect) regularity, the mathematics
developed for gratings apply well. Conversely, for random wires
(even if they all have the same orientation) the theory of
scattering provides the best description. Scattering from a single
cylindrical wire has been described (H. C. Van de Hulst, Light
Scattering by Small Particles, Dover, 1981). The current
random-wire grids have wires embedded throughout the substrate. Not
only are the positions of the wires somewhat random, but the
diameters are as well. It is clear that the phases of the scattered
rays will be random, so the reflection will not be strictly
specular and the transmission will not retain high spacial or image
fidelity. Such degradation of the light beam would prevent its use
for transfer of well resolved, high-information density images.
Nothing in the prior art indicates or suggests that an ordered
array of wires can or should be made to operate over the entire
visible range as a spathic PBS, at least at the angles required
when it serves both as a polarizer and analyzer. Indeed, the
difficulty of making the narrow, tall, evenly spaced wires that are
required for such operation has been generously noted (see Zeitner,
et. al. Applied Optics, 38, 11 pp. 2177-2181 (1999), and Schnabel,
et. al., Optical Engineering 38,2 pp. 220-226 (1999)). Therefore,
it is not surprising that the prior art for image projection
similarly makes no suggestion for use of a spathic PBS as part of a
display device.
Tamada and Matsumoto (U.S. Pat. No. 5,748,368) disclose a wire grid
polarizer that operates in both the infrared and a portion of the
visible spectrum; however, it is based on the concept that large,
widely spaced wires will create resonance and polarization at an
unexpectedly short wavelength in the visible. Unfortunately, this
device works well only over a narrow band of visible wavelengths,
and not over the entire visible spectrum. It is therefore not
suitable for use in producing images in full color. Accordingly,
such a device is not practical for image display because a
polarizer must be substantially achromatic for an image projection
system.
Another reason wire grid polarizers have been overlooked is the
common and long standing belief that the performance of a typical
wire grid polarizer becomes degraded as the light beam's angle of
incidence becomes large (G. R. Bird and M. Parrish, Jr., "The Wire
Grid as a Near-Infrared Polarizer," J. Opt. Soc. Am., 50, pp.
886-891, (1960); the Handbook of Optics, Michael Bass, Volume II,
p. 3-34, McGraw-Hill (1995)). There are no reports of designs that
operate well for angles beyond 35.degree. incidence in the visible
portion of the spectrum. Nor has anyone identified the important
design factors that cause this limitation of incidence angle. This
perceived design limitation becomes even greater when one realizes
that a successful beam splitter will require suitable performance
in both transmission and reflection simultaneously.
This important point deserves emphasis. The extant literature and
patent history for wire grid polarizers in the IR and the visible
spectra has almost entirely focused on their use as transmission
polarizers, and not on reflective properties. Wire grid polarizers
have been attempted and reported in the technical literature for
decades, and have become increasingly common since the 1960s.
Despite the extensive work done in this field, there is very
little, if any, detailed discussion of the production and use of
wire grid polarizers as reflective polarizers, and nothing in the
literature concerning their use as both transmissive and reflective
polarizers simultaneously, as would be necessary in a spathic
polarizing beam splitter for use in imaging devices. From the lack
of discussion in the literature, a reasonable investigator would
conclude that any potential use of wire grid polarizers as
broadband visible beam splitters is riot apparent, or that it was
commonly understood by the technical community that their use in
such an application was not practical.
Because the conventional polarizers described above were the only
ones available, it was impossible for Takanashi (U.S. Pat. No.
5,239,322) to reduce his projection device to practice with
anything but the most meager results. No polarizer was available
which supplied the performance required for the Takanashi
invention, namely, achromaticity over the visible part of the
spectrum, wide angular acceptance, low losses in transmission and
reflection of the desired light polarizations, and good extinction
ratio.
There are several important features of an image display system
which require specialized performance of transmission and
reflection properties. For a projector, the product of
p-polarization transmission and s-polarization reflection (R.sub.s
T.sub.p) must be large if source light is to be efficiently placed
on the screen. On the other hand, for the resolution and contrast
needed to achieve high information density on the screen, it is
important that the converse product (R.sub.p T.sub.s) be very small
(i.e. the transmission of s-polarized light multiplied by the
reflection of p-polarized light must be small).
Another important feature is a wide acceptance angle. The
acceptance angle must be large if light gathering from the source,
and hence the conversion efficiency, is maximized. It is desirable
that cones of light (either diverging or converging) with
half-angles greater than 20.degree. be accepted.
An important consequence of the ability to accept larger light
cones and work well at large angles is that the optical design of
the imaging system is no longer restricted. Conventional light
sources can be then be used, bringing their advantages of low cost,
cool operation, small size, and low weight. A wide range of angles
makes it possible for the designer to position the other optical
elements in favorable positions to improve the size and operation
of the display.
Another important feature is size and weight. The conventional
technology requires the use of a glass cube. This cube imposes
certain requirements and penalties on the system. The requirements
imposed include the need to deal with thermal loading of this large
piece of glass and the need for high quality materials without
stress birefringence, etc., which impose additional cost. In
addition, the extra weight and bulk of the cube itself poses
difficulties. Thus, it is desirable that the beam splitter not
occupy much volume and does not weigh very much.
Another important feature is robustness. Modern light sources
generate very high thermal gradients in the polarizer immediately
after the light is switched on. At best, this can induce thermal
birefringence which causes cross talk between polarizations. What
is more, the long duration of exposure to intense light causes some
materials to change properties (typically yellowing from
photo-oxidation). Thus, it is desirable for the beam splitter to
withstand high temperatures as well as long periods of intense
radiation from light sources.
Still another important feature is uniform extinction (or contrast)
performance of the beam splitter over the incident cone of light. A
McNeille-type thin film stack polarizer produces polarized light
due to the difference in reflectivity of S-polarized light as
opposed to P-polarized light. Since the definition of S and P
polarization depends on the plane of incidence of the light ray,
which changes orientation within the cone of light incident on the
polarizer, a McNeille-type polarizer does not work equally well
over the entire cone. This weakness in McNeille-type polarizers is
well known. It must be addressed in projection system design by
restricting the angular size of the cone of light, and by
compensation elsewhere in the optical system through the use of
additional optical components. This fundamental weakness of
McNeille prisms raises the costs and complexities of current
projection systems, and limits system performance through
restrictions on the f-number or optical efficiency of the beam
splitter.
Other important features include ease of alignment. Production
costs and maintenance are both directly affected by assembly
criteria. These costs can be significantly reduced with components
which do not require low tolerance alignments.
Therefore, it would be advantageous to develop an image projection
system capable of providing bright images and good image contrast,
and which is inexpensive. It would also be advantageous to develop
an image projection system with a polarizing beam splitter capable
of utilizing divergent light (or having a smaller F/#), capable of
efficient use of light energy or with high conversion efficiency,
and which is durable. It would also be advantageous to develop an
image projection system with a polarizing beam splitter having a
high extinction ratio, uniform response over the visible spectrum,
good color fidelity, that is spathic, robust and capable of
resisting thermal gradients. It would also be advantageous to
develop an image projection system with a polarizing beam splitter
capable of being positioned at substantially any incidence angle so
that significant design constraints are not imposed on the image
projection system, but substantial design flexibility is permitted.
It would also be advantageous to develop an image projection system
with a polarizing beam splitter which efficiently transmits
p-polarized light and reflects s-polarized light across all angles
in the entire cone of incident light. It would also be advantageous
to develop an image projection system with a polarizing beam
splitter which is light-weight and compact. It would also be
advantageous to develop an image projection system with a
polarizing beam splitter which is easy to align. Combining all of
these features in a single projection device would offer a
significant advance within the state of the art.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an
image projection system which provides bright images with good
image contrast, and which is inexpensive.
It is another object of the present invention to provide an image
projection system with a polarizing beam splitter which utilizes
divergent light (or has a smaller F/#), efficiently uses light
energy, has good conversion efficiency, and is durable.
It is another object of the present invention to provide an image
projection system with a polarizing beam splitter with a high
extinction ratio, a uniform response over the visible spectrum,
good color fidelity, which is spathic, robust and resists thermal
gradients.
It is another object of the present invention to provide an image
projection system with a polarizing beam splitter capable of
selectively directing either or both of the transmitted and
reflected polarized beams at substantially any angle.
It is yet another object of the present invention to provide an
image projection system with a polarizing beam splitter which
functions adequately while positioned with respect to the source
light beam at substantially any incident angle.
It is yet another object of the present invention to provide an
image projection system with a polarizing beam splitter which
efficiently transmits p-polarized light and reflects s-polarized
light over all angles within the cone of light, but can also
function to transmit s-polarized light and reflect p-polarized
light similarly.
It is yet another object of the present invention to provide an
image projection system with a polarizing beam splitter which is
light-weight, compact, robust, and easy to align.
It is a further object of the present invention to provide a
polarizing beam splitter for use in image projection systems.
These and other objects and advantages of the present invention are
realized in an image projection system with a polarizing beam
splitter which advantageously is a wire grid polarizer. The wire
grid polarizing beam splitter has a generally parallel arrangement
of thin, elongated elements. The arrangement is configured, and the
elements are sized, to interact with electromagnetic waves of the
source light beam to generally transmit one polarization of light
through the elements, and reflect the other polarization from the
elements. Light having a polarization oriented perpendicular to a
plane that includes at least one of the elements and the direction
of the incident light beam is transmitted, and defines a
transmitted beam. The opposite polarization, or light having a
polarization oriented parallel with the plane that includes at
least one of the elements and the direction of the incident light
beam, is reflected, and defines a reflected beam.
The system includes a light source for producing a mostly
unpolarized, visible light beam. The polarizing beam splitter is
located proximal to the light source in the light beam. The system
also includes a reflective liquid crystal array. The array may be
located proximal to the polarizing beam splitter in either the
reflected or transmitted beam. The array modulates the polarization
of the beam, and creates a modulated beam. The array is oriented to
direct the modulated beam back to the beam splitter. The
arrangement of elements of the beam splitter interacts with
electromagnetic waves of the modulated beam to again generally
transmit one polarization and reflect the other polarization. Thus,
the reflected portion of the modulated beam defines a second
reflected beam, while the transmitted portion defines a second
transmitted beam. The array alters the polarization of the beam to
encode image information on the modulated beam. The beam splitter
separates the modulated polarization from the unmodulated beam,
thus making the image visible on a screen.
A screen is disposed in either the second reflected or second
transmitted beam. If the array is disposed in the reflected beam,
then the screen is disposed in the second transmitted beam. If the
array is disposed in the transmitted beam, then the screen is
disposed in the second reflected beam.
Unlike the bulky, heavy beam splitters of the prior art, the beam
splitter of the present invention is a generally planar sheet. The
beam splitter is also efficient, thus providing greater luminous
efficacy of the system.
In accordance with one aspect of the present invention, the beam
splitter is capable of being oriented with respect to the light
beam and the modulated beam at incidence angles between
approximately 0 to 80 degrees.
In accordance with another aspect of the present invention, the
light beam has a useful divergent cone with a half angle between
approximately 12 and 25.degree.. The beam splitter is used at a
small F-number, preferably between approximately 1.2 and 2.5.
In accordance with another aspect of the present invention, the
beam splitter has a conversion efficiency of at least 50% defined
by the product of the s-polarization reflected light and the
p-polarization transmitted light (R.sub.s T.sub.p). In addition,
the s-polarization transmitted light and the p-polarization
reflected light are both less than 5%. Furthermore, the percentage
of reflected light and the percentage of the transmitted light of
the modulated beam is greater than approximately 67%.
In accordance with another aspect of the present invention, the
system may include a pre-polarizer disposed between the light
source and the beam splitter, and/or a post-polarizer disposed
between the beam splitter and the screen.
These and other objects, features, advantages and alternative
aspects of the present invention will become apparent to those
skilled in the art from a consideration of the following detailed
description taken in combination with the accompanying
drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1a is a schematic view of the general operation of a preferred
embodiment of an image projection system of the present invention
using a wire grid polarizing beam splitter of the present
invention.
FIGS. 1b and 1c are schematic views of the image projection system
of the present invention in different configurations.
FIG. 2a is a graphical plot showing the relationship between
wavelength and transmittance for both S and P polarizations of a
preferred embodiment of the wire grid polarizing beam splitter of
the present invention.
FIG. 2b is a graphical plot showing the relationship between
wavelength and reflectance for both S and P polarizations of a
preferred embodiment of the wire grid polarizing beam splitter of
the present invention.
FIG. 2c is a graphical plot showing the relationship between
wavelength, efficiency and transmission extinction of a preferred
embodiment of the wire grid polarizing beam splitter of the present
invention.
FIG. 3 is a graphical plot showing the performance of the preferred
embodiment of the wire grid polarizing beam splitter of the present
invention as a function of the incident angle.
FIG. 4a is a graphical plot showing the theoretical throughput
performance of an alternative embodiment of the wire grid
polarizing beam splitter of the present invention.
FIG. 4b is a graphical plot showing the theoretical extinction
performance of an alternative embodiment of the wire grid
polarizing beam splitter of the present invention.
FIG. 4c is a graphical plot showing the theoretical extinction
performance of an alternative embodiment of the wire grid
polarizing beam splitter of the present invention.
FIG. 5a is a schematic view of the general operation of an
alternative embodiment of an image projection system of the present
invention.
FIGS. 5b and 5c are schematic views of the image projection system
of the present invention in different configurations.
FIG. 6 is a schematic view of the general operation of an
alternative embodiment of an image projection system of the present
invention.
FIG. 7 is a perspective view of the wire grid polarizing beam
splitter of the present invention.
FIG. 8 is a cross sectional side view of the wire grid polarizing
beam splitter of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Reference will now be made to the drawings in which the various
elements of the present invention will be given numerical
designations and in which the invention will be discussed so as to
enable one skilled in the art to make and use the invention.
As illustrated in FIG. 1a, a display optical train of an image
projection system of the present invention, indicated generally at
10, is shown. The image projection system 10 advantageously has a
wire grid polarizer as the beam splitter, indicated generally at
14. The wire grid polarizing beam splitter 14 (WGP-PBS) efficiently
reflects light of one polarization from a source 20 to a reflective
liquid crystal array 26, and then efficiently transmits reflected
light of the opposite polarization to a display screen 25.
For adequate optical efficiency, the WGP-PBS 14 must have high
reflectivity (R.sub.s) of the desired polarization from the light
source 20, and it must have high transmissivity (T.sub.p) of the
opposite polarization from the liquid crystals array 26. The
conversion efficiency is proportional to the product of these two,
R.sub.s T.sub.p, so deficiency in one factor can be compensated to
some extent by improvement in the other.
Examples of wire grid polarizing beam splitters 14 of the present
invention advantageously show the following characteristics which
demonstrate the advantage of using a WGP-PBS 14 of the present
invention as both the polarizer and analyzer in display devices for
the visible portion of the spectrum. Theoretical calculations of
further improvements indicate that even better polarizing beam
splitters will be available.
Referring to FIGS. 2a and 2b, the measured transmissivity and
reflectivity, respectively, for both S and P polarizations of a
WGP-PBS are shown. In FIG. 2c, the efficiency of the WGP-PBS is
shown as the product of the transmissivity and reflectivity. In
addition, the extinction is also shown in FIG. 2c. In FIGS. 2a-2c,
the WGP-PBS is oriented to reflect the s-polarization and transmit
the p-polarization at incident angles of 30.degree., 45.degree. and
60.degree.. For an image projection system, such as a projector,
the product of the reflected s-polarization and transmitted
p-polarization (R.sub.s T.sub.p) must be large if source light is
to be efficiently placed on the screen. On the other hand, for the
resolution needed to achieve high information density on the
screen, it is important that the converse product (R.sub.p T.sub.s)
be very small (i.e. the transmission of s-polarized light
multiplied by the reflection of p-polarized light must be small).
It is clear from the figures that the wire grid polarizing beam
splitter of the present invention meets these standards over the
entire spectrum without degradation by Rayleigh resonance or other
phenomena.
Another important feature is a wide acceptance angle. This must be
large if light gathering from the source, and hence the conversion
efficiency, is maximized. Referring to FIG. 3, the performance of
the wire grid polarizing beam splitter of the present invention is
shown for various portions of the light cone centered around the
optical axis which is inclined at 45.degree.. In FIG. 3, the first
referenced angle is the angle in the plane of incidence while the
second referenced angle is the angle in the plane perpendicular to
the plane of incidence. It is clear that the WGP-PBS of the present
invention is able to accept cones of light (either diverging or
converging) with half-angles between approximately 12 and
25.degree..
Referring to FIGS. 4a-4c, theoretical calculations for an
alternative embodiment of a wire grid polarizing beam splitter
indicate that significantly larger light cones and/or other
enhancements will be possible. FIGS. 4a and 4b show the theoretical
throughput and extinction, respectively, of a wire grid polarizing
beam splitter with a period p reduced to 130 nm. In addition, the
grid height or thickness is 130 nm; the line-spacing ratio is 0.48;
the substrate groove depth is 50 nm; and the substrate is BK7
glass. It should be noted in FIG. 4a that the throughput is grouped
much more closely than the throughput shown in FIG. 2c. Therefore,
performance can be improved by reducing the period p. It should be
noted in FIG. 4b that the extinction is significantly increased in
comparison to FIG. 2c.
FIG. 4c shows the theoretical extinction of another alternative
embodiment of the wire grid polarizing beam splitter with the
period p further reduced. The wavelength is 420 nm and the
incidence angle is 30.degree.. It should be noted that the
extinction increases markedly as the period p is reduced.
As indicated above, an important consequence of the ability to
accept larger light cones with a WGP-PBS that will work well at
large angles is that the PBS no longer restricts the optical design
of the imaging system. Thus, conventional light sources can be
used, with the advantage of their low cost, cooler operation, small
size, and low weight. The wide range of angles over which the
WGP-PBS works well makes it possible for the designer to position
the other optical elements in favorable positions to improve the
size and operation of the display. Referring to FIGS. 1b and 1c,
the design flexibility provided by the wide range of angles of the
PBS of the present invention is demonstrated. As shown in FIG. 1b,
the light source 20 and array 26 may be positioned closer together,
with both having a relatively small incident angle with respect to
the PBS 14. Such a configuration is advantageous for a compact
design of the components of the system 10. Alternatively, as shown
in FIG. 1c, the light source 20 and array 26 may be positioned
farther apart, with both having a relatively large incident angle.
In should be noted that in either case, the incidence angles vary
greatly from the 45 degree angle typically required by traditional
beam splitters.
Yet other features of wire grids provide advantages for display
units. The conventional technology requires the use of a glass
cube. This cube imposes certain requirements and penalties on the
system. The requirements imposed include the need to deal with
thermal loading of this large piece of glass, the need for high
quality materials without stress birefringence, etc., which impose
additional cost, and the extra weight and bulk of the cube itself.
The WGP-PBS of the present invention advantageously is a divided or
patterned thin film that does not occupy much volume and does not
weigh very much. It can even be integrated with or incorporated
into other optical elements such as color filters, to further
reduce part count, weight, and volume of the projection system.
The WGP-PBS of the present invention is also very robust. Modern
light sources generate very high thermal gradients in the polarizer
immediately after the light is switched on. At best, this can
induce thermal and stress birefringence which causes cross talk
between polarizations. At worst, it can delaminate multilayer
polarizers or cause the cemented interface in a cube beam splitter
to separate. What is more, the long duration of exposure to intense
light causes some materials to change properties (typically
yellowing from photo-oxidation). However, wire grid polarizing beam
splitters are made of chemically inert metal that is well attached
to glass or other substrate materials. They have been shown to
withstand high temperatures as well as long periods of intense
radiation from light sources.
The WGP-PBS of the present invention also is easy to align. It is a
single part that needs to be adjusted to direct the source beam
onto the liquid crystal array. This is the same simple procedure
that would be used for a flat mirror. There is another adjustment
parameter, namely, the angular rotation about the normal to the WGP
surface. This determines the orientation of polarization in the
light beam. This adjustment is not critical because the WGP
functions as its own analyzer and cannot be out of alignment in
this sense. If there are other polarizing elements in the optical
train, the WGP-PBS should be oriented with respect to their
polarization, but slight misalignment is not critical because:
according to Malus' law, angular variation makes very little
difference in the intensity transmitted by polarizers if their
polarization axes are close to being parallel (or
perpendicular).
In order to be competitive with conventional polarizers, the
product R.sub.s T.sub.p must be above approximately 50%. This
represents a lower estimate which would only be practical if the
WGP-PBS was able to gather significantly more light from the light
source than the conventional polarizing beam splitters. The
estimate of 50% comes from an assumption that the best conventional
beam splitter, a modern MacNeille cube beam splitter, can deliver
an f/# of about f/2.5 at best. An optical system which was twice as
fast, or capable of gathering twice as much light, would then have
an f/# of 1/2 of this value, or about f/1.8, which is certainly a
reasonable f/# in optical image projection systems. A system which
is twice as fast, and therefore capable of gathering twice the
light from the source, would approximately compensate for the
factor of 2 decrease in the R.sub.s T.sub.p product over the
conventional cube beam splitter, resulting in an equivalent
projection system performance. In fact, since a WGP-PBS can
potentially be used down below f/1.2 (a factor of four increase)
this seemingly low limit can still produce very bright images of
course, an R.sub.s T.sub.p product which is over this minimum value
will provide even better performance.
Another important performance factor is contrast in the image, as
defined by the ratio of intensities of light to dark pixels. One of
the significant advantages of the WGP-PBS is the improved contrast
over compound incident angles in comparison to the prior art cube
beam splitter such as a McNeille prism. The physics of the McNeille
prism polarizes light by taking advantage of the difference in
reflectivity of S vs. P polarization at certain angles. Because S
and P polarization are defined with respect to the plane of
incidence, the effective S and P polarization for a particular ray
in a cone of light rotates with respect to the ray along the
optical axis as various rays within the cone of light are
considered. The consequence of this behavior is the well-known
compound angle problem in which the extinction of the polarizer is
significantly reduced for certain ranges of angles within the cone
of light passing through the polarizing beam splitter,
significantly reducing the average contrast over the cone.
The WGP-PBS, on the other hand, employs a different physical
mechanism to accomplish the polarization of light which largely
avoids this problem. This difference in behavior is due to the fact
that the polarization is caused by the wire grids in the beam
splitter which have the same orientation in space regardless of the
plane of incidence for any particular ray in the cone of light.
Therefore, even though the plane of incidence for any particular
ray is the same when incident on a McNeille prism or a WGP, the
polarization effect is only dependent on the plane of incidence in
the case of the McNeille prism, meaning the compound angle
performance of the WGP is much improved over that provided by the
cube beam splitter.
The fact that the function of the WGP-PBS is independent of the
plane of incidence means that the WGP-PBS can actually be used with
the wires or elements oriented in any direction. The preferred
embodiment of the invention has the elements oriented parallel to
the axis around which the polarizer is tilted so that the light
strikes the WGP-PBS at an angle. This particular orientation is
preferred because it causes the polarization effects of the surface
reflections from the substrate to be additive to the polarization
effects from the grid. It is possible, however, to produce a
WGP-PBS which functions to reflect the P-polarization and transmit
the S-polarization (which is exactly opposite what has been
generally described herein) over certain ranges of incident angles
by rotating the grid elements so they are perpendicular to the tilt
axis of the WGP-PBS. Similarly, the grid elements can be placed at
an arbitrary angle to the tilt axis to obtain a WGP-PBS which
functions to transmit and reflect light with polarizations aligned
with the projection of this arbitrary angle onto the wavefront in
the light beam. It is therefore clear that WGP-PBS which reflect
the P-polarization and transmit the S-polarization, or which
reflect and transmit light with polarization oriented at arbitrary
angles are included within this invention.
The compound angle performance advantage of the WGP-PBS provides an
inherently more uniform contrast over the entire light cone, and is
one of the reasons the WGP is suitable for very small f-numbers.
But, of course, it is not the only factor affecting the image
contrast. The image contrast is governed to a large extent by low
leakage of the undesired polarization, but in this case the product
T.sub.s R.sub.p is not the important parameter, because the image
generating array which lies in sequence after the first encounter
with the beam splitter but before the second also takes part in the
production of the image contrast. Therefore, the final system
contrast will depend on the light valve performance as well as the
polarizer extinction. However, lower bounds on the required beam
splitter performance can be determined with the assumption that the
light valve performance is sufficient enough that it can be assumed
to have an essentially infinite contrast. In this case, the system
contrast will depend entirely on the beam splitter performance.
Referring to FIG. 1a, there are two different functions fulfilled
by the beam splitter 14. The first is the preparation of the
polarized light before it strikes the liquid crystal array 26 or
other suitable image generation device. The requirement here is
that the light be sufficiently well polarized that any variations
in the polarization of the light beam created by the light valve
can be adequately detected or analyzed such that the final image
will meet the desired level of performance. Similarly, the beam
splitter 14 must have sufficient performance to analyze light which
is directed by the light valve back to the beam splitter so that
the desired system contrast performance is achieved.
These lower bounds can be determined fairly easily. For reasons of
utility and image quality, it is doubtful that an image with a
contrast of less than 10:1 (bright pixel to adjacent dark pixel)
would have much utility. Such a display would not be useful for
dense text, for example. If a minimum display system contrast of
10:1 is assumed, then an incident beam of light is required which
has at least 10 times the light of the desired polarization state
over that of the undesired polarization state. In terms of
polarizer performance, this would be described as having an
extinction of 10:1 or of simply 10.
The second encounter with the beam splitter 14 which is going to
analyze the image, must be able to pass the light of the right
polarization state, while eliminating most of the light of the
undesired state. Again, assuming from above a light beam with an
image encoded in the polarization state, and that this light beam
has the 10:1 ratio assumed, then a beam splitter is desired which
preserves this 10:1 ratio to meet the goal of a system contrast of
10:1. In other words, it is desired to reduce the light of the
undesired polarization by a factor of 10 over that of the right
polarization. This again leads to a minimum extinction performance
of 10:1 for the analysis function of the beam splitter.
Clearly, higher system contrast will occur if either or both of the
polarizer and analyzer functions of the beam splitter have a higher
extinction performance. It is also clear that it is not required
that the performance in both the analyzer function and the
polarizer function of the beam splitter be matched for a image
projection system to perform adequately. An upper bound on the
polarizer and analyzer performance of the beam splitter is more
difficult to determine, but it is clear that extinctions in excess
of approximately 20,000 are not needed in this application. A good
quality movie projection system as found in a quality theater does
not typically have an image contrast over about 1000, and it is
doubtful that the human eye can reliably discriminate between an
image with a contrast in the range of several thousand and one with
a contrast over 10,000. Given a need to produce an image with a
contrast of several thousand, and assuming that the light valves
capable of this feat exist, an upper bound on the beam splitter
extinction in the range of 10,000-20,000 would be sufficient.
The above delineation of the minimum and maximum bounds on the wire
grid beam splitter is instructive, but as is clear from the
demonstrated and theoretical performance of a wire grid beam
splitter as shown above, much better than this can be achieved. In
accordance with this information, the preferred embodiment has
R.sub.s T.sub.p.gtoreq.65%, and R.sub.p or T.sub.p or both are
.gtoreq.2 67%, as shown in FIGS. 2a-2c. The preferred embodiment
would also employ the wire grid polarizing beam splitter in the
mode where the reflected beam is directed to the image generating
array, with the array directing the light back to the beam splitter
such that it passes through, or is transmitted through, the beam
splitter. This preferred embodiment is shown in FIG. 1a.
Alternatively, as shown in the image display system 60 of FIG. 5a,
the wire grid polarizing beam splitter 14 may efficiently transmit
light of one polarization from the source 20 to the reflective
liquid crystal array 26, and then efficiently reflect the reflected
light of the opposite polarization to the display screen 25. The
second embodiment of the image projection system 60 is similar to
that of the preferred embodiment shown in FIG. 1a, with the
exception that the beam splitter 14 would be employed in a manner
in which the source beam of light is transmitted or passed through
the beam splitter 14 and directed at the image generating array 26,
then is reflected back to the beam splitter 14 where it is
reflected by the beam splitter and analyzed before being displayed
on the screen 25.
Again, referring to FIGS. 5b and 5c, the design flexibility
provided by the wide range of angles of the PBS of the present
invention is demonstrated. As shown in FIG. 5b, the array 26 and
screen 25 may be positioned closer together, with both having a
relatively small incident angle with respect to the PBS 14.
Alternatively, as shown in FIG. 5c, the array 26 and screen 25 may
be positioned farther apart, with both having a relatively large
incident angle.
As shown in FIG. 6, a third embodiment of image projection system
80 provides an alternative system design which may assist in
achieving a desired level of system performance. This third
embodiment would include one or more additional transmissive or
reflective polarizers which work in series with the wire grid
polarizing beam splitter to increase the extinction of either or
both of the polarizing and analyzing functions to achieve the
necessary system contrast performance. Another reason for
additional polarizers would be the implementation of a polarization
recovery scheme to increase the system efficiency. A pre-polarizer
82 is disposed in the source light beam between the light source 20
and the WGP-PBS 14. A post-polarizer or clean-up polarizer 84 is
disposed in the modulated beam, or the beam reflected from the
array 26, between the array 26 and the screen 25, or between the
WGP-PBS 14 and the screen 25. The third embodiment would still
realize the advantages of the wire grid beam splitter's larger
light cone, durability, and the other advantages discussed
above.
As shown in the figures, the image display system may also utilize
light gathering optics 90 and projection optics 92.
Referring to FIGS. 7 and 8, the wire grid polarizing beam splitter
14 of the present invention is shown in greater detail. The
polarizing beam splitter is further discussed in greater detail in
co-pending U.S. application Ser. No. 09/390,833, , filed Sep. 7,
1999, and now allowed, which is herein incorporated by
reference.
As described in the co-pending application, the polarizing beam
splitter 14 has a grid 30, or an array of parallel, conductive
elements, disposed on a substrate 40. The source light beam 130
produced by the light source 20 is incident on the polarizing beam
splitter 14 with the optical axis at an angle .theta. from normal,
with the plane of incidence preferably orthogonal to the conductive
elements. An alternative embodiment would place the plane of
incidence at an angle .theta. to the plane of conductive elements,
with .THETA. approximately 45.degree.. Still another alternative
embodiment would place the plane of incidence parallel to the
conductive elements. The polarizing beam splitter 14 divides this
beam 130 into a specularly reflected component 140, and a
transmitted component 150. Using the standard definitions for S and
P polarization, the light with S polarization has the polarization
vector orthogonal to the plane of incidence, and thus parallel to
the conductive elements. Conversely, light with P polarization has
the polarization vector parallel to the plane of incidence and thus
orthogonal to the conductive elements.
Ideally, the polarizing beam splitter 14 will function as a perfect
mirror for the S polarized light, and will be perfectly transparent
for the P polarized light. In practice, however, even the most
reflective metals used as mirrors absorb some fraction of the
incident light, and thus the WGP will reflect only 90% to 95%, and
plain glass does not transmit 100% of the incident light due to
surface reflections.
The key physical parameters of the wire grid beam splitter 14 which
must be optimized as a group in order to achieve the level of
performance required include: the period p of the wire grid 30, the
height or thickness t of the grid elements 30, the width w of the
grids elements 30, and the slope of the grid elements sides. It
will be noted in examining FIG. 8 that the general cross-section of
the grid elements 30 is trapezoidal or rectangular in nature. This
general shape is also a necessary feature of the polarizing beam
splitter 14 of the preferred embodiment, but allowance is made for
the natural small variations due to manufacturing processes, such
as the rounding of corners 50, and fillets 54, at the base of the
grid elements 30.
It should also be noted that the period p of the wire grid 30 must
be regular in order to achieve the specular reflection performance
required to meet the imaging fidelity requirements of the beam
splitter 14. While it is obviously better to have the grid 30
completely regular and uniform, some applications may have relaxed
requirements in which this is not as critical. However, it is
believed that a variation in period p of less than 10% across a
meaningful dimension in the image (such as the size of a single
character in a textual display, or a few pixels in an image) is
required to achieve the necessary performance.
Similarly, reasonable variations across the beam splitter 14 in the
other parameters described, such as the width w of the grid
elements 30, the grid element height t, the slopes of the sides, or
even the corner rounding 50, and the fillets 54, are also possible
without materially affecting the display performance, especially if
the beam splitter 14 is not at an image plane in the optical
system, as will often be the case. These variations may even be
visible in the finished beam splitter 14 as fringes, variations in
transmission efficiency, reflection efficiency, color uniformity,
etc. and still provide a useful part for specific applications in
the projection imaging system.
The design goal which must be met by the optimization of these
parameters is to produce the best efficiency or throughput
possible, while meeting the contrast requirements of the
application. As stated above, the minimum practical extinction
required of the polarizing beam splitter 14 is on the order of 10.
It has been found that the minimum required throughput (R.sub.s
T.sub.p) of the beam splitter 14 in order to have a valuable
product is approximately 50%, which means either or both of Rp and
Ts must be above about 67%. Of course, higher performance in both
the throughput and the extinction of the beam splitter will be of
value and provide a better product. In order to understand how
these parameters affect the performance of the wire grid beam
splitter, it is necessary to examine the variation in performance
produced by each parameter for an incident angle of 45.degree., and
probably other angles of interest.
The performance of the wire grid beam splitter 14 is a function of
the period p. The period p of the wire grid elements 30 must fall
under approximately 0.21 .mu.m to produce a beam splitter 14 which
has reasonable performance throughout the visible spectrum, though
it would be obvious to those skilled in the art that a larger
period beam splitter would be useful in systems which are expected
to display less than the full visible spectrum, such as just red,
red and green, etc.
The performance of the wire grid beam splitter 14 is a function of
the element height or thickness t. The wire-grid height t must be
between about 0.04 and 0.5 .mu.m in order to provide the required
performance.
The performance of the wire grid beam splitter 14 is a function of
the width to period ratio (w/p) of the elements 30. The width w of
the grid element 30 with respect to the period p must fall within
the ranges of approximately 0.3 to 0.76 in order to provide the
required performance.
The performance of the wire grid beam splitter 14 is a function of
the slopes of the sides of the elements 30. The slopes of the sides
of the grid elements 30 preferably are greater than 68 degrees from
horizontal in order to provide the required performance.
It is to be understood that the described embodiments of the
invention are illustrative only, and that modifications thereof may
occur to those skilled in the art. For example, the inclusion of
the wire grid beam splitter on a substrate with optical power, such
that the grid beam splitter is combined or integrated with other
elements to reduce the number of optics required, the system
weight, the system volume, or to achieve other desirable
attributes. Other alterations will surely occur to those skilled in
the art given the significant increase in design flexibility over
the prior art that is achieved by the present invention.
Accordingly, this invention is not to be regarded as limited to the
embodiments disclosed, but is to be limited only as defined by the
appended claims herein.
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